1. Field of the Invention
The present invention relates to the field of differential pressure transducers, and, in particular, it relates to improvements in differential pressure transducers of the type employing piezoresistive strain gauge elements on a force-responsive cantilever beam.
2. Description of the Prior Art
Various differential pressure transducers are known in which a rigid rod or wire connects a pressure-sensitive diaphragm to a cantilever beam having piezoresistive strain gauges mounted thereon. Examples of such devices are disclosed in the following U.S. Pat. Nos.: 4,161,887 to Stone et al.; 4,203,327 to Singh; 4,058,788 to Andrews et al.; 3,559,488 to Weaver; 3,389,362 to McLellan; and 3,161,844 to Kabell.
In such devices, the motion of the diaphragm, in response to a pressure differential applied across it, is transmitted, via the rigid connecting link, to the beam, causing the latter member to flex. The strain gauges are typically electrically connected in a Wheatstone bridge configuration, such that the flexing of the beam causes a change in the output voltage of the bridge circuit due to the piezoresistive qualities of the strain gauges. This output voltage, finally, is fed into associated signal processing circuitry, well-known in the art, which provides a reading of the sensed differential pressure.
The typical prior art transducer, of the type described above, utilizes a beam which is relatively stiff, or noncompliant, as compared with the sensing diaphragm. The advantage of such a design is that the resulting small full scale deflection of the beam provides a relatively high signal-to-deflection ratio, so that adequate signal quality is achieved for relatively small beam deflections. While such a design is acceptable for measuring relatively high differential pressures, the low deflection-to-force ratio of such devices yields a low signal-to-force ratio which renders such devices less than optimal for low differential pressure ranges, i.e., on the order of 1 to 5 psi.
Moreover, in such prior art devices, a high degree of care must be taken to thermally match, as precisely as possible, all pressure-sensitive and pressure-responsive components and links, since ambient temperture effects which produce even a small beam deflection result in degraded signal accuracy. In short, such devices, without precise thermal matching of components, do not display a relatively high signal-to-noise ratio due to ambient temperature effects. The requirement for such thermal matching increases the cost of such prior art devices.
Furthermore, in order to protect such devices in overpressure situations, the components must be constructed to very close tolerances. Thus, for example, if overpressure stops are set at twice the full scale deflection, and the full scale deflection is small, the tolerances in the overpressure stop mechanism must be proportionately close to maintain the required overpressure function.
Finally, in prior art devices, using a stiff beam/compliant diaphragm combination, the measurable differential pressure range is determined by the physical characteristics (i.e., the stiffness) of the beam. Such devices are typically operative only over a relatively narrow pressure range. Therefore, different pressure ranges can be accommodated only by changing the beam to one with a different stiffness. This factor contributes to the expense and inconvenience of utilizing such devices in different pressure ranges, since the beam is usually the most expensive component, as well as the most difficult to replace. Thus, the usual approach is to have several transducers on hand, each designed for optimal operation in a different pressure range, where a wide variety of pressure ranges is expected.
Underlying the need for a low compliance (stiff) beam, in combination with a compliant diaphragm in the prior art devices, is the fact that the beam in such devices is structured so that the bending action of the beam, in response to the force on the diaphragm, is distributed throughout the length of the beam, and is not concentrated in the area containing the strain gauge elements. The result is a substantial loss in efficiency, up to about 50 percent. Thus, such devices display a relatively low signal-to-force ratio, necessitating a design in which diaphragm forces are accumulated, or collected, on the beam, in order to obtain an adequate strain gauge response. This criterion is achieved through use of a diaphragm which is highly compliant as compared with the beam. The beam, being relatively stiff, exhibits a high degree of strain for a given degree of deflection, so that the aforementioned high signal-to-deflection ratio is achieved. Thus, the prior art devices achieve adequate strain gauge response through a mechanism which depends upon low compliance beam, resulting in the nonoptimal low pressure range performance, along with the other limitations in the stiff beam design, mentioned above.
There is thus a need in the art for a differential pressure transducer which achieves adequate signal quality over a wide range of differential pressures, including very low differential pressures, and which achieves accuracy and overpressure tolerance.
Moreover, there is a need for a device which displays such capabilities in a design which lends itself to economies in production costs. Finally, there is a need for such a device which achieves near-perfect operational efficiencies.